Home Backup Generator Sizing Calculator

Running and locked-rotor demand

Motor electrical running power is shaft power divided by efficiency. Apparent running power is $S_r=P_r/\mathrm{PF}_r$ and the quadrature component is $Q_r=\sqrt{S_r^2-P_r^2}$. Sum real and quadrature components separately, then take their vector magnitude; do not add unlike power factors as scalar kVA.

For each motor, $S_{LR}=K_{code}H$ with $H$ in mechanical horsepower and the user-entered $K_{code}$ in kVA/hp. The locked-rotor convention is described by Baldor-Reliance with reference to NEMA MG 1-2011, §10.37.2. This calculator reproduces no code-letter table. Current is $1000S_{LR}/V$ for single phase or $1000S_{LR}/(\sqrt3V)$ for balanced three phase.

Start states and sizing

Background demand is already connected. Each motor contributes zero before its start, its locked-rotor complex power for the entered duration, and its running complex power afterward. Evaluating every start and end instant finds the exact maximum of this rectangular envelope, even with overlap. Animation time only reads the cached sizing results.

Cummins T-030, Issue 9 (2015), §7.1.9 discusses largest-motor-first sequencing and typically 5–10 s stabilization between load steps. Real controls must enforce the order; recurring automatic restarts may coincide.

The additive site factor is $D=1-d_hh/304.8-d_T\Delta T/5.6$, with altitude in metres and $\Delta T=\max(T-25\,^{\circ}\mathrm C,0)$ in K. Default slopes follow the Champion 201249 operator manual, rev 20251223. Apply the same factor conservatively to the kW and kVA capacity limits; replace the assumptions with the more restrictive manufacturer site limits.

For each limit, calculate the required continuous nameplate kW. Take the greatest of real and apparent running demand with reserve, peak starting real and apparent demand without overload credit, and the electrical dip requirement.

Voltage screening model derived here

Represent the connected loads by rated-voltage equivalent impedances and the source by fixed 1 pu internal voltage behind purely inductive reactance $X$. With nameplate base $S_g=P_g/\mathrm{PF}_g$, the terminal voltage fraction is

For required fraction $v_0$, let $a=v_0^2$. Solving the quadratic gives

Use the largest value across load states. Reactance must be on the selected generator's nameplate kVA base. Site derating affects capacity, not the electrical reactance base. This voltage-divider derivation is our screening model, not a Cummins performance curve. Cummins T-030 §§4.2.5.6–4.2.5.8, printed pp. 45–47, explain why initial dip and subsequent recovery require different equipment data.

Part-load fuel and ownership cost

Fit $F=a_fP_g+b_fP_{out}$ from two measured reference loads. The slope is the fuel-flow difference divided by output-power difference; the intercept is normalized by the reference generator rating. This is HOMER's linear fuel-curve form. The Honda endpoint checks reproduce the calibration data; they do not independently validate predictions between those points or scaling to other generators. Require a strictly positive idle intercept for a running engine. Use every positive fuel-test output interval exactly; reject a zero or negative interval. Average fuel uses all-on real demand times the average-load fraction; sizing uses the full demands.

Monthly hours are daily operating hours times outage days. Multiply hours by fuel flow and dated unit price. Annual operation repeats the planning month twelve times. Cumulative costs include purchase, fuel, maintenance, generator replacement by operating hours, and battery replacement by calendar years. The crossover is the first year-end where solar-plus-battery cumulative cost is no higher; later replacements can reverse it.

• Conventional synchronous generator, balanced single/split phase or balanced three phase. All listed motors use the entered line-to-line voltage. Split-phase leg limits need a separate check.
• Three direct-on-line induction-motor equivalents at rated shaft load. A common illustrative locked-rotor PF of 0.35 and the entered rectangular duration describe starting states, not acceleration. Motor efficiency and running PF are editable planning assumptions.
• Equivalent impedances are held constant within each state. Reported starting kVA and current are nominal-voltage quantities. No credit is taken for reduced current during sag when checking capacity.
• Fixed internal source voltage, no automatic voltage regulator, governor or frequency dynamics. The electrical envelope can retain a running-load drop after inrush ends. It is not a measured waveform or a guaranteed worst-case transient bound.
• Default reactance 0.12 pu is an illustrative assumption, not a selected generator's specification. The separate Honda fuel calibration is not a Honda voltage-dip model; Honda EU7000iS is an inverter set.
• Default fuel points use 5.1 US gal / 16.0 h at quarter load and 5.1 US gal / 6.4 h at full rated 5.5 kW. The Honda checks verify reproduction of these calibration endpoints and their published tank runtimes, not independent predictive accuracy. Scaling normalized fuel coefficients between generators is approximate. Actual size, control mode, fuel and site conditions can change them. Idle is an extrapolation; an off engine consumes zero, while the entered operating hours describe an on engine.
• Price baseline: EIA 2025 annual US regular gasoline, 3.097 USD/US gal = approximately 0.81814 USD/L, checked 2026-10-25. The USD/L price field is a currency label and stays per litre under the unit switch.
• All purchase, maintenance, service-life and replacement defaults are illustrative budgets, not quotations. Constant prices, undiscounted cash flows, no residual value, grants, financing or grid-export savings. Replacements are charged at the year-end in which an interval is reached, including the horizon end.
• The alternative solar-plus-battery installation must independently meet the same outage energy, motor starts and reliability target. Its energy production, battery capacity and autonomy are not calculated here.

Use this as a preliminary selection and cost-comparison screen, then verify a real generator with its manufacturer. Check against the standard in force; the cited historical NEMA edition describes the code-letter convention, not an installation approval.

Input ranges cover household and small commercial examples, but do not certify every combination. The additive derating model is rejected below 30% retained capacity. The generator's actual engine and alternator limits, fuel specification and altitude adaptation govern. No site uprating is credited below 25 °C or at sea level.

Not applicable to current-limited inverter-generator voltage response, VFD or soft-start input harmonics, highly unbalanced split-phase circuits, large constant-power electronic loads, feeder voltage drop, fault protection, motor torque and thermal limits, or medical and other life-safety service. Reduced voltage reduces induction-motor starting torque approximately with voltage squared; passing the dip screen alone does not establish acceleration.

Enforce the selected sequence with suitable controls. Check All together for uncontrolled coincident restarts. Enter fuel points that imply strictly positive idle consumption for the running engine. The two Honda endpoint checks are calibration checks; no withheld measured operating point validates the interpolated or scaled curve. Use the fuel curve within its measured load range and for the actual measured generator. Fuel properties and part-load controls can make the real curve nonlinear.

CO: portable generators outdoors, at least 6.1 m (20 ft) from openings, exhaust directed away, working CO alarms. Back-feed: never use a wall outlet; approved transfer equipment must isolate the utility. Installation work, circuit loading, grounding, bonding and protection belong to a qualified electrician following local rules and manufacturer instructions.